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There's a selection on that screen for "last known good configuration" - try that, sometimes a driver that has been recently installed can cause a machine to crash on boot. If that doesn't help, you can attempt to boot from the installation CD that came with the computer and repair the system. Boot from the install CD, select ENTER in order to continue with the setup program (trust me, we're not going to do a full setup, but you have to choose the first option here...). More files will load from the CD - the next screen should be the repair screen - it should say "To repair the selected Windows XP installation, press R" - press R, and the repair should begin. Reboot after it's done.

Run a restore point to before this happened (IF Possible) and reboot. If not, you will need to enter in the exact error message into Google and follow instructions for going into registry manually and fix that way.